The invention described herein may be manufactured, used, and licensed by or for the government for governmental purposes without the payment of any royalty thereon.
This invention relates to an improved multi-layer insulation for extreme-temperature apparatus. In this respect, the invention will be described primarily in connection with a Dewar vessel, but it will be understood that the invention applies to other extreme-temperature devices such as superconducting magnet enclosures, cryogenic piping and the like. Generally speaking, such apparatus includes an inner container, an outer container, an evacuated space therebetween, and radiation barriers located in the vacuum space.
Structures with which the invention finds particular utility experience three types of heat transfer, namely, radiation, gas conduction (convection), and solid conduction. For a given Dewar vessel, for example, it has been customary to reduce radiation heat transfer by installing a plurality of radiation barriers in the vacuum space. Hence, the description "multi-layer insulation" (MLI) has been applied. Gas conduction is reduced by evacuation, and solid conduction has been reduced by various mechanical means.
For constant surface emissivities in a reasonably good vacuum (10.sup.-5 Torr or lower), heat leak generally decreases with 1/N where N is the number of reflective shields between the vessel's warm and cold surfaces. To obtain adequate performance, however, it is necessary that the reflective shields be prevented from touching each other. Otherwise, thermal shorts occur. Hence, separator materials are required to keep the multiple radiation shields from touching. Otherwise, the separator materials only add to the vessel's heat transfer rate. In this respect, a typical installation might include one or more sheets of separator material between reflective shields of thin aluminum foil or double-aluminized Mylar (DAM).
Often, "bumper strips" are installed between the reflective layers. These bumper strips might cover from 10-15% of the insulation area. Such bumper strips improve performance by increasing spacing without a proportional increase of solid conduction as compared to simply adding more separator sheets, but they are not as efficient as the industry would like. Hence, it is an object of this invention to provide improved separating structures which will retain the benefits of the customary reflective barriers while reducing the solid conduction contributed by the separator materials and reducing the overall heat transfer between a Dewar vessel's inner and outer walls.